A polyimide resin is a useful engineering plastic that has high thermal stability, high strength and high solvent resistance due to rigidity, resonance stabilization and firm chemical bond of the molecular chain thereof, and is being applied to a wide range of fields. A polyimide resin having crystallinity is further enhanced in the heat resistance, the strength and the chemical resistance thereof, and thus is expected for applications as alternatives of metals or the like. While a polyimide resin has high heat resistance, however, it has the problems of exhibiting no thermoplasticity and having low molding process ability.
Vespel (registered trademark), a highly heat-resistant resin, is known as a polyimide molding material (PTL 1). This resin is difficult to process by molding due to its very low flowability even at a high temperature, and is also disadvantageous in terms of cost because it requires molding under conditions of a high temperature and a high pressure for a prolonged period of time. In contrast to this, a resin having a melting point and flowability at a high temperature, such as a crystalline resin, may be processed by molding easily and inexpensively.
Thus, a polyimide resin having thermoplasticity has been reported in recent years. Such a thermoplastic polyimide resin is excellent in molding processability in addition to the original heat resistance of the polyimide resin. The thermoplastic polyimide resin is therefore applicable to a molded article for use in an inhospitable environment to which nylon or polyester, a general purpose thermoplastic resin, is inapplicable.
Aurum (registered trademark) or the like is known as a thermoplastic polyimide resin (NPL 1). Aurum is, however, limited in terms of an available apparatus because of having a high melting point and requiring a molding temperature of generally 400° C. or more.
A method using a long linear aliphatic diamine as a raw material diamine is one of the methods for improving the molding processability of the polyimide resin, i.e., the methods for decreasing the melting point of the polyimide resin (NPL 2). This reduces the rigidity of the polyimide resin, and thus also decreases the melting point. This method, however, might decrease the glass transition temperature along with the decrease of the melting point, and in particular, might reduce the strength at a high temperature. Another problem of this method is difficult synthesis of a polyimide resin using a raw material diamine composed mainly of an aliphatic diamine.
In view of the above problems, a polyimide resin satisfying both molding processability and heat resistance and a composition comprising the polyimide resin are developed (PTLs 5 and 6).